The present invention relates to a control circuit intended to control the flow of a current through a coil of an electric motor in order to produce a magnetic field which sets the motor into motion, said coil being coupled to an output terminal of the circuit, which circuit is arranged between a positive supply terminal and a negative supply terminal and comprises:
a power transistor of the NMOS type arranged between the negative supply terminal and the output terminal, and PA1 a power transistor of the PMOS type arranged between the output terminal and the positive supply terminal, said transistor being implanted in a well with an N-type diffusion, which is electrically coupled to the positive supply terminal, the conduction of the power transistors of the PMOS type and of the NMOS type being controlled by complementary signals.
Such control circuits are generally used for the control of devices utilizing information storage discs, particularly hard discs for information processing equipment. These devices usually have a first motor, which serves to impart a rotary movement to the disc, and a second motor, which serves to move an arm carrying one or more heads for reading/writing information from the surface of the disc. Each motor has input terminals, called supply terminals, which are coupled to coils intended to produce a magnetic field by means of which the motor can be driven. Each of the supply terminals is coupled to the output terminal of a control circuit. In the event of a sudden power failure, i.e. when the positive supply voltage becomes zero while the device is in operation, the rotor of the first motor continues to rotate, driven by its kinetic energy, and produces an electromotive force across the supply terminals of this motor, which then operates as a generator. This electromotive force, which takes the place of the supply voltage of the second motor, should allow the motor to move the arm towards a disc zone, called parking zone, which has been provided to allow the arm to be parked there without damage being caused to the read/write heads.
However, it has been found that the PN junction existing between the drain of a PMOS power transistor and the well in which this transistor is implanted forms a parasitic diode. When the voltage on the positive supply terminal is zero, the well being usually coupled to the source, which itself is coupled to this positive supply terminal, said parasitic diode short-circuits the motor terminal to which it is coupled via the output terminal of the control circuit, thereby allowing a substantial leakage current to flow directly to a terminal at zero potential. This prevents the electromotive force produced by the first motor from being used for energizing the second motor. Various solutions have been proposed to solve this problem, which essentially comprise the insertion of a power component by means of which the leakage current between the source of the PMOS transistor and the positive supply terminal can be blocked in one direction. For example, in the solution described in the publication ISBN 3-540-60332-8 "Smart Power ICs" by SGS-Thomson, a power diode has its cathode coupled to the source of the PMOS transistor and has its anode coupled to the positive supply terminal. This solution has two major drawbacks: such a power component is expensive and a substantial voltage drop is produced across it. Indeed, the value of the supply voltage in the normal mode of operation is currently of the order of 5 V and constantly much development effort is deployed to reduce this value with a dual general purpose of saving energy and reducing the size of electronic circuits. Since a voltage drop of the order of 0.5 V is produced across the power diode in a most advantageous case where the diode is of the Schottky type, this diode reduces the useful power supplied to the motor and, consequently, the energy efficiency of the device by at least 10%, which adverse effect of the power diode on the efficiency is bound to increase as the value of the supply voltage decreases.